In all modules import Prelude explicitly and replace every occurrence of Prelude with UTF8Prelude.

Use the NoImplicitPrelude language extension.

In all modules replace every occurrence of System.IO with System.UTF8IO.

utf8-prelude also provides a basic testing tool:
utf8-test tests the functions readFile, writeFile and putStrLn.
Note: utf8-test creates a file named writeFile_test.txt in the current working directory.